You can find all the information about this mod on the RockBand forums. Another member has also posted a guide:
This mod would not have been possible without some great friends on the Rockband forums.
Highly Liquid has posted about the mod:
Aelius for getting the ball rolling.
Ben & Greg for their soldering prowess.
Zolon for finding the midi relay...
and Highly_Liquid for making it
AND updating the firmware just for us!
Rockband forum thread: